In certain types of printing operations, particularly those using what is known as the sublimation process, a transfer sheet is prepared with patterns formed on it of the ink to be used in the printing operation. The transfer sheet is laid on the surface to be printed and heat and pressure is applied to the transfer sheet to vaporize the ink forming the patterns and cause the patterns to become permanently embedded in the surface to be printed. One of the particularly useful applications for sublimation printing is to place the characters and other patterns on the keys of keyboards for desktop and laptop computers.
It is particularly efficient to print all of the keys for a single keyboard simultaneously with a transfer sheet arranged so that individual patterns correspond to the final position of the pattern in the keyboard. The blank keys are carried in some kind of alignment support which may be the actual keyboard frame itself when presented to the apparatus which performs the actual printing operation. In such a case, it is important that the patterns on the transfer sheet are precisely alined with the keytops when the printing occurs. To aid in this alignment, there are registration pins carried in the printing apparatus with respect to which both the alignment support and the transfer sheet are positioned. It is convenient to provide registration apertures in the transfer sheet spaced apart from the patterns themselves and through which pass the registration pins when the transfer sheet is placed in contact with the keytops to thereby allow the transfer sheet to be aligned with the keytops.
As anyone who has access to a personal computer keyboard can easily determine, the maximum horizontal distance between two keytops on such a keyboard can easily be in excess of 16 in. (37 cm.). The paper or other material from which the transfer sheet is formed is not dimensionally stable when subjected to humidity or temperature changes, and its dimension can in a typical production situation change in length as much as 5 to 10 parts per thousand. Thus it is possible that patterns at one or another part of the keyboard may be out of alignment as much as 0.15 inch, or nearly 0.4 cm. Such misalignment is unacceptable. Presently, such out of alignment keyboards are reworked or discarded which is expensive and inconvenient. There is therefore a need to correct this misalignment in some way.
This characteristic of paper has been recognized in the past. For example U.S. Pat. No. 4,870,434 uses a detector to sense the edge of each segment of a fanfold paper stack to assure that each segment is printed in accurate alignment with the edge even if the paper has changed length due to effects of humidity or temperature.
It is also known to provide grooves in articles such as flooring panels to provide relief of stresses arising from absorption of moisture, see U.S. Pat. No. 4,390,580.